In Drosophila melanogaster there exists a class of mutations called "Minutes," which is characterized by phenotypic abnormalities caused by defective protein synthesis. While several of these loci have been identified as encoding ribosomal proteins, others are likely to encode other components of the protein synthesizing apparatus. The Minute (1)1B [M(1)1B] gene is likely a member of the latter category, in that its putative protein product has a molecular weight of approximately 125 KD, much larger than the largest ribosomal polypeptide (approximately 50 KD). From cloning and sequencing the M(1)1B gene, we have identified an open reading frame that could encode a 1133 amino acid polypeptide. This protein has no homology to any known protein sequences or to proteins that could be produced by the GenEMBL nucleic acid database. The only similarity to any known functional protein motif is the occurrence of a "leucine zipper" near the middle of the polypeptide chain. Our goal is to identify the cellular function of this protein.